Glass articles can be extensively found in homes, offices and factories in the form of lenses, prisms, mirrors, CRT tubes, flat display glass, vehicle windshields, computer disc substrates, furniture glass, art glass and the like. The grinding, finishing and polishing of these types of glass objects to an optical clarity is of utmost importance. If present, defects, imperfections, and even minute scratches can inhibit the optical clarity of the glass article and can even inhibit the ability to accurately see through the glass. Thus, it is desired that the glass be essentially free of any defects, imperfections, scratches and be optically clear.
Many optical components contain some type of curve or radius associated with the glass articles. There are several different means to generate a curve/radius on a glass surface. One means is to use abrasive articles and in general, there are three main processes for such shape generation: rough grinding, fining and polishing.
Rough Grinding
The first step is to generate the desired curve or radius by rough grinding the optical component with an abrasive tool. Typically this abrasive tool includes a super-hard abrasive particle such as a diamond, tungsten carbide or cubic boron nitride. The resulting glass surface is usually of the approximate curvature required. The abrasive tool in this rough grinding process will impart course scratches into the glass surface such that the resulting glass surface is neither precise enough nor smooth enough to directly polish to an optically clear state.
Fining
The purpose of the fining step is to refine the coarse scratches generated by the rough grinding process. In general, the fining process removes the deep scratches remaining after rough grinding and provides a substantially smooth, although not polished surface. The fining process should also result in sufficient removal of the coarse scratches such that the glass surface can be polished to an optically clear surface. If the fining process does not remove all the coarse scratches, then it can be extremely difficult for the polishing step to remove these scratches to generate an optically clear surface. In the case of ophthalmic lenses, this fining process is typically done in the presence of a liquid medium such as water, with a conventional coated abrasive article, a lapping coated abrasive article or a combination of conventional coated abrasive and lapping coated abrasive articles. The conventional coated abrasive article includes a backing having a first binder layer, commonly referred to as a make coat, applied over the backing. A plurality of abrasive particles are at least partially embedded into the make coat. Over the abrasive particles/make coat is a second binder layer, commonly referred to as a size coat and this size coat reinforces the abrasive particles. A lapping coated abrasive article includes a backing having an abrasive coating bonded to the backing. This abrasive coating comprises a plurality of abrasive particles dispersed in a binder. There is at least one fining step, typically two or more fining steps, with each subsequent fining step utilizing an abrasive article that contains a smaller or finer abrasive particle size than the previous step. In the case of other glass surfaces, such as CRT tube glass, this fining is typically done with abrasive slurries. Previous attempts in using fixed abrasive articles have, in general, been unsuccessful on a wide scale commercial basis.
The first fining step usually uses abrasive particles with an average particle size of 15 to 40 micrometers depending upon the surface finish produced by the rough grinding step. The second fining step usually uses abrasive particles at least about 50% finer than the first, usually 4 to 12 micrometers. The time required for the two fining steps is usually from about one minute to two minutes per step, depending on the starting surface finish, the abrasive particle size and the desired surface finish. The surface finish of the optical component after this fining process is typically anywhere from about 0.06 to 0.13 micrometer (Ra) and/or an Rtm greater than about 0.40 to 1.4 micrometer.
The roughness of a surface is typically due to scratches or a scratch pattern, which may or may not be visible to the naked eye. A scratch pattern can be defined as a series of peaks and valleys along the surface. Rtm is a common measure of roughness used in the abrasives industry, however, the exact measuring procedure can vary with the type of equipment utilized in surface roughness evaluation. As used herein, Rtm measurements are based on procedures followed with the Rank Taylor Hobson profilometer, available under the trade designation SURTRONIC 3. Within the Rank Taylor Hobson purview, Rt is defined as the maximum peak-to-valley height within an assessment length set by the Rank Taylor Hobson instrument. Rtm is the average, measured over five consecutive assessment lengths, of the maximum peak-to-valley height in each assessment length. Rtm is measured with a profilometer probe, which is a 5 micrometer radius diamond tipped stylus and the results are recorded in micrometers (.mu.m). In general, the lower the Rtm value, the smoother the finish. A slight variation in the absolute Rtm value can, but not necessarily, occur when the measurement on the same finished glass surface is performed on different brands of commercially available profilometers.
Ra is defined as an average roughness height value of an arithmetic average of the departures of the surface roughness profile from a mean line on the surface, also measured in micrometers (.mu.m).
Polishing
The third step is the polishing step which generates the optically clear surface on the glass article. In many instances, this polishing step is done with a loose abrasive slurry. Loose abrasive slurries typically comprise a plurality of very fine abrasive particles (i.e., less than about 10 micrometers, usually less than about 5 micrometers) dispersed in a liquid medium such as water. The loose abrasive slurry may optionally contain other additives such as dispersants, lubricants, defoamers and the like. Loose abrasive slurries are usually the preferred means to generate the final polish because of the ability of the loose abrasive slurries to remove essentially all the remaining scratches to generate an optically clear surface that is essentially free of any defects, imperfections and/or minute scratches.
It is well recognized that small differences in Rtm or Ra values can have a significant impact on the clarity of the polished glass surface, i.e., a small difference in Rtm can mean the difference between an optically clear surface and a hazy surface. The input finish to final polishing (i.e., to optical clarity) can also vary widely depending upon the process. For example, starting finishes prior to polishing might have Ra values from about 0.05 to about 0.2 micrometers and Rtm values from about 1.0 to about 2.0 micrometers. Other values outside these ranges may also be encountered prior to polishing.
Polishing machinery utilized depends largely upon the application and the material being polished. For example, ophthalmic lenses may be polished utilizing polishing machines such as a Coburn 5000 or a Coburn 5056 cylinder machine or a Coburn 507 flat lapping machine, all available from Coburn Optical Industries Inc., Muskogee, Okla. These machines rely on a fixed motion, which may be orbital or a figure 8 type motion, of abrasive material while the lens is swept over the abrasive. Pressures of about 35 kPa (5 psi) to about 350 kPa (50 psi) might be used, however, pressures from about 70 kPa (10 psi) to about 210 kPa (30 psi) are typical. The fixed abrasive in this case could have a, so-called, daisy configuration so that the fixed abrasive pad is capable of conforming to a curved polishing arm so that there are no creases or folds in the fixed abrasive pad.
For example, EPO Publication No. 650803 to Lindholm et al. discloses a method for polishing an optical quality surface, such as an ophthalmic lens using abrasive composites, without an abrasive slurry. Essentially all abrasive particles eroded from the abrasive composites are removed from the interface between the surface to be polished and the abrasive article. Erosion of abrasive particles from the abrasive composites brings a continuous supply of new abrasive particles in the abrasive composites into engagement with the first major surface. Thus, polishing is substantially accomplished by the abrasive particles held in the binder, not the eroded abrasive particles.
CRT face panels are currently ground and finished on large rotary hemispherical lappers, utilizing various types of abrasive slurries and pads. The final polish step (i.e., to optical clarity) typically utilizes a ceria slurry on a segmented felt pad. The slurry is pumped on to the pad-glass panel interface. Industrial flat lapping of computer thin film disc wafers is accomplished much the same way by using an precision flat lap (having a diameter from 12 to 60 inches) rather than the hemispherical lap with the abrasive slurry.
Loose abrasive slurries are widely utilized in the final polishing steps of glass articles, however, many disadvantages are associated with them. These disadvantages include the inconvenience of handling the required large volume of the slurry, the required agitation to prevent settling of the abrasive particles and to assure a uniform concentration of abrasive particles at the polishing interface, and the need for additional equipment to prepare, handle, and also recover and recycle the loose abrasive slurry. Additionally, the slurry itself must be periodically analyzed to assure its quality and dispersion stability which requires additional costly man hours. Furthermore, pump heads, valves, feed lines, grinding laps, and other parts of the slurry supply equipment which contact the loose abrasive slurry eventually show undesirable wear. Further, during usage, the polishing operation is usually very untidy because the loose abrasive slurry, which is usually applied as a viscous liquid to a soft pad, splatters easily and is difficult to contain.
Understandably, attempts have been made to replace the loose abrasive slurry polishing systems with lapping coated abrasives to some degree of success. For example, U.S. Pat. Nos. 4,255,164 (Butzke et al.), 4,576,612 (Shukla et al.) and 4,733,502 (Braun) disclose various abrasive articles and polishing processes. Other references that teach lapping coated abrasive articles include U.S. Pat. Nos. 4,644,703 (Kaczmarek et al.); 4,773,920 (Chasman et al.) and 5,014,468 (Ravipati et al.). However, lapping coated abrasives have not commercially replaced loose abrasive slurries. In some instances the lapping coated abrasives do not completely polish the glass article so that the resulting surface is optical clear and essentially free of defects, imperfections and minute scratches. In other instances, the lapping coated abrasives require a longer time to polish the glass article, thereby it is more cost effective to use a loose abrasive slurry.
Much less technical industrial glass is polished offhand. This process typically utilizes a 7 to 12 inch diameter felt buff wheel mounted on a backstand grinder. A ceria-based slurry or compound polish are the abrasives typically used in offhand polishing. Rotational speeds are generally from about 500 to about 1500 rpm with applied pressures of about 70 kPa (10 psi) to about 420 kPa (60 psi). Additionally, random scratches in glass are also removed by offhand polishing using right angle grinders mounting 5 to 10 inch pads with ceria slurries or compounds. As explained above, slurry-based polishing methods exhibit significant disadvantages.
Further, in the past, polishing a glass workpiece has typically required operator training prior to efficient use of currently available polishing matrices, such as abrasive slurries and lapping films. Operator training is of importance because the operator's technique affects polishing matrix breakdown, which results in the liberation of abrasive particles. A slow initial breakdown time results in slower polishing rates. This phenomenon has resulted in poor consumer acceptance of a polishing method using an abrasive cerium oxide pad product from their current methods using buffing pads and ceria slurries or pastes.
Therefore, there is a need for a more user friendly, durable, and rapid method for polishing optical quality glass surfaces, which obviates the need to use an external abrasive slurry or a gel polishing techniques but instead utilizes a fixed abrasive article, thus eliminating time required to polish to optical clarity and reduce the mess generated by the polishing procedure. It is further desired by the glass industry that an abrasive article does not exhibit the disadvantages associated with a loose abrasive slurry yet it is able to effectively polish a glass surface in a reasonable time to optical clarity such that the glass surface is essentially free of imperfections, defects and scratches. Additionally, there exists a need to supply a fixed abrasive article which rapidly breaks down, resulting in faster initial polishing rates and yet has a polishing life at least equal to other polishing pad matrices.